A woman whose husband and two sons are fighting for their lives from the incident says she found them “flaked out” in the tank.
The three men became ill as the father used an ammonium cleaning agent to clean a tank that contained stockfeed.
Seven others who went to help, including two police officers and at least one paramedic, were affected by the fumes at the Cloverlea property, near Warragul, about 104km south-east of Melbourne.
The father and sons were flown to Melbourne hospitals in critical condition.
Cheryl Coster said family and friends cut a hole in the side of the tank to rescue her husband and sons.
“By the time I got up and looked in, three of them flaked out in the bottom of it,” she told reporters.
“I was up on the tank yelling `help, help, help’ and on the phone for help.”
Air Ambulance operations manager Paul Holman says it is believed the emergency unfolded as one of the three most seriously affected men was cleaning inside a tank with ammonia.
“He was overcome, possibly not so much by the fumes but by a lack of oxygen, while another two men were also overcome,” he said.
Mr Holman said all three men were very sick when they were flown to Melbourne hospitals.
Hospital staff said all three remained in critical conditions.
The seven people injured while offering aid were taken to the Warragul hospital in serious but stable conditions.
Mr Holman said they suffered from nausea due to the smell of the chemical.